America hasn't always been like this. "The US will never start a war," the president said. "We do not want a war. This generation of Americans has already had enough of war and hate and oppression. We shall be prepared if others wish it. We shall be alert to try to stop it. But we shall do our part to build a world of peace where the weak are safe and the strong are just."

This would not be "a Pax Americana enforced on the world by American weapons of war". The time was June 1963, and John F Kennedy was talking "about genuine peace, the kind of peace that makes life on earth worth living. Not the peace of the grave or the security of the slave, but the kind that enables men and nations to grow and to hope and to build a better life for their children - not merely peace for Americans but peace for all men and women. There is no single key to this peace, no grand or magic formula to be adopted by one or two powers. Genuine peace must be the product of many nations, the sum of many acts".

Kennedy could speak these words 40 years ago, and find an audience that believed them. They articulated a credible ideal, infused with internationalist generosity. Lurking beneath the horrors of today, that ideal still in some places exists. There are Americans who believe this is what is going on as their country takes command of the new world order. American democracy and freedom will make the world a better place, as the world will soon understand. Some suppose the end of Saddam will produce Iraqi democracy, which will set up a liberal model for the Middle East, which will begin to settle every problem in the region.

What we're learning is that this is fantasy at every level, beginning with the official rhetoric. Not a trace of Kennedy's internationalism has ever crossed the lips of George Bush. During the cold war, Kennedy could sincerely conjure up visions of peace which Bush, as leader of the hegemon, positively rejects. In place of peace as the product of many nations we have a marginalised UN, its legitimacies scorned, its existence challenged by officials close to the heart of power in Washington. It is inconceivable that Bush would make a speech disclaiming the merits of a Pax Americana.

The neoconservatism out of which he comes revels in such a Pax, or at least the illusion of it. Whether on arms control, environmental regulation or international criminal law, Americana has unilaterally ruled. The ideology of national interest, for the most part, trumps even a pretence that what Washington does might have selfless consequences for the good of the world. Unchallengeable military power, instead of generating magnanimity, has produced only a more righteous determination to use it.

It's true that this coexists with an ideology of democracy. From top to bottom, Americans do believe democracy is good for everyone, even if some may have to wait for it longer than others. But here comes the crux of the American dilemma. Even if we're prepared to grant the existence, deep in American purposes, of more idealism than is usually admitted, its fulfilment has become unattainable. America's understanding of the world has become so self-centred, and its reputation so corrupted, that its ability to export liberal democracy either by example or by force now looks to be non-existent.

The Iraq war is one proof of this. Whether or not Bush and Co thought Saddam's army would collapse, they certainly believed that the majority of Iraqis would show simple delight at the prospect of liberation. This has not happened, and now looks entirely remote rather than being a merely temporary offshoot of Saddam's reign of terror. What Iraq is experiencing is invasion and bombing, which, as the people see it, destroys their security and challenges their self-respect: factors that Americans, wedded to the superiority of their own system, cannot comprehend.

What Iraqis see, and the world along with them, is a hegemon going about its business of domination, and barely any longer interested in why it is hated for doing so. Its motives, to put them no lower, are compromised. If it doesn't want the oil for itself, it doesn't want any other country - Russia, France - to have any either. If it doesn't want the burden of reconstructing Iraq itself, it resists any similar power for the UN or the EU. Under the guise of advocating democracy, it works to preserve its dominance in all respects. Its interests must be paramount. Its system and no other must prevail, irrespective of culture, history and the rights of sovereign nations.

What Kennedy said of communism, in the same 60s address, could be transposed, with uncanny accuracy, to Americanism today. "The communist drive to impose their political and economic system on others," he contended, "is the primary cause of world tension today. For there can be no doubt that, if all nations could refrain from interfering in the self-determination of others, the peace would be much more assured." The role reversal may not be exact. World terrorism has to be factored into the equation. But as a verdict on Bush's America, this picture of political and economic imperialism rampant helps explain why a peaceful new world order seems out of reach.

There is only one Kennedyesque figure in the Bush entourage, and his name is Tony Blair. Blair, at a moral level, is fired by something of the same imperialism. He is the great intervener, proud, even now, to say that if Bush had held back from intervening in Iraq he would have been pushing him in that direction. An extraordinary, revealing confession to hear at this dire and misbegotten time.

But at least Blair's motives are not compromised. Like Kennedy he sees the role of power as being to work, sometimes, for nationally disinterested purposes. He's an internationalist visionary, albeit a naive one. He believes he was put on earth to make it a better place, in ways that have little to do with the power or riches of his own country.

This, however, makes him look the more forlorn. It marks one more divide between him and the mighty ally whose armed fig leaf he has allowed this country to become. He is tainted, much as he might dislike it, by American political strategies. Around the world, he is seen the way Bush's neocons are seen, even though many people, especially in Europe, are mystified about how and why he allowed this to happen. He has been sucked into their power games, their world view, and their grotesque insensitivity to the interests and judgments of other nations.

He has, maybe, one final chance to break free of this fealty. The next test is over who does construct the political authority in postwar Iraq. Blair believes, allegedly with passion, that it has to be the UN. Looking at the scale of the problem, most people would agree that internationalisation is the only way. Does our leader have the nerve to speak and vote with Kennedy, not Bush?